The present invention concerns a method for the compensation of rotor torque harmonics, and an apparatus for carrying out the method.
Double-fed asynchronous machines are used preferably in variable-speed systems, for example in high-power wind power installations, as a wave generator or in conjunction with flywheel mass storage means and uninterruptible power supplies. In general in such installations the stator of the asynchronous machine is connected to the mains and the rotor is connected to a converter by way of slip rings. With such a converter a reference value of an electrical parameter can be impressed into the rotor. In general modern installations have a machine-side converter and a mains-side converter which are connected together by way of an intermediate circuit. The advantage of the double-fed asynchronous machine over comparable systems lies in the reduced building work involved in the converter in comparison with the total power which can be fed into a mains. That provides that a system having a double-fed asynchronous machine has a comparatively high level of efficiency.
In the course of the increasing number of wind power installations and the demands of the mains operators, which are linked thereto and which are ever increasing, the performance of the double-fed asynchronous machine in the event of mains asymmetries is increasingly of interest. As, in a double-fed asynchronous machine, the stator of the machine is connected directly to the mains, mains asymmetries result in asymmetric stator field distributions, this leading to unwanted reactions on the rotor field and the converters.
In the event of an asymmetric mains voltage in particular harmonics occur, especially harmonic oscillations at double the mains frequency, which have a detrimental effect in terms of the mechanism as the harmonic oscillations generate corresponding torque oscillations which in particular heavily load the transmission.
Besides the harmonic oscillations at double the mains frequency, which are primarily discussed hereinafter, unwanted higher harmonics can also occur—generally at lower amplitude—; this primarily involves the sixth and twelfth harmonics.